The present invention relates generally to power conversion systems and more particularly to a system for controlling the current supplied to a synchronous machine such as an AC motor from a polyphase alternating current source.
Many circuits and systems are known for controlling the conductivity of controlled rectifiers utilized in various types of converters for supplying electrical power to a load such as an AC motor, from a polyphase alternating current (AC) source. The type of rectifier used controls to some degree the type of control utilized, but by far the most common controlled rectifier in use today is a thyristor of the silicon controlled rectifier type which becomes conductive with the simultaneous application of a forward bias voltage and a signal applied to a gate electrode and which thereafter remains conductive until the anode current falls below the value required to hold the thyristor in the conductive state.
In U.S. Pat. No. 4,230,979, entitled, "Controlled Current Inverter And Motor Control System", Paul M. Espelage, et al. which issued on Oct. 28, 1980, there is disclosed a system which forms the basis of an AC motor drive for furnishing a synchronous motor with a variable frequency, variable magnitude AC current from a thyristor controlled load side converter or inverter which is supplied from a thyristor controlled source or line side converter by way of a direct current (DC) link including an inductor. Means are included to develop signals representing the instantaneous electrical torque of the AC motor and the instantaneous air gap power factor from which electrical control signals drive the line side converter to control the DC current in the link while the firing angle of the inverter is driven with respect to the motor flux such that the angle is maintained substantially constant over its prescribed operating range.
With respect to a load commutated inverter for a three phase (3.phi.) AC motor drive, a typical example of which is shown and described in U.S. Pat. No. 4,276,505, entitled, "Microcomputer Based Control Apparatus For A Load-Commutated Inverter Synchronous Machine And Drive System", Bimal K. Bose, June 30, 1981, control of the inverter supplying the synchronous motor is normally based upon a thyristor firing strategy wherein the firing of the thyristors is provided at or near the commutation limit point, i.e. at a power factor angle just sufficiently leading to provide the necessary volt-seconds necessary to safely commutate the current transfer from one thyristor to the other of the thyristor bridge implementing the inverter. The Bose patent, moreover, discloses a control system which is implemented by digital techniques including a microcomputer having software programs which comprise coded instructional sets for effecting the necessary control routines from the sensed operational parameters. Both the Bose patent and the Espelage, et al. patent are specifically intended to be incorporated herein by reference for better enabling one skilled in the art to understand the present invention without necessity of the disclosure of extraneous and irrelevant material.
In load commutated inverter drives such as noted above, the basic operation is such that the line side power converter is controlled to regulate motor current through the DC link circuit while the load side converter is commanded to produce motoring or braking torque, with both of these control channels being responsive to a signal generated by a speed control regulator. The control of such a system, however, is lost whenever the line side converter is unable to maintain control of the DC link current which can happen in a number of ways. For example, when a line disturbance occurs on the polyphase AC input line, the value of the line voltage dips. In some instances the dip can be large enough to cause a reduction in the line side DC voltage which appears at the output of the line side converter and one side of the link circuit while the DC voltage at the input side of the load converter and the other side of the link circuit is larger than that capable from the line side converter. In such a condition, in a motoring mode the current in the link circuit will be reduced to zero. Even though the line side converter senses this condition, it cannot drive the current into the load side converter. In a braking mode while the opposite situation exists, control is also lost with current larger, rather than smaller than desired.
Another condition can exist where control is lost at high speed under a no load condition of the motor. There the motor voltage which exists on the line between the motor and the load side converter rises to the extent that it can exceed the AC line voltage. In such a situation, the line side converter is unable to generate a DC voltage which is greater than the load voltage when motoring is again required, causing current to fall in the DC link to zero and control to be lost.
One known solution to the aforementioned control problem is to oversize the line voltage by as much as 20 to 30% above that of the unloaded motor at top speed. This acts to lessen the occurrence where the DC voltage into the load side converter is greater than the voltage at the output of the line side converter, but lessens the capability of the line side converter and provides an inherent undesirable limitation on system operation, efficiency, and line power factor.